Asked by: Andrew Richardson, Durham
Galaxies are certainly ‘born’ and ‘evolve’, but they don’t have a clearly defined ‘life cycle’ like stars do, for example. In the early Universe, ‘proto-galaxies’ were formed from tiny variations in the density of matter in space. As the proto-galaxies collapsed, the matter began to spin faster, thus flattening the proto-galaxies into thin discs. Within the discs, the density was high enough for stars to form and the first spiral galaxies were born.
Since then, galaxies have evolved mainly due to the gravitational interactions between them. The merger of two spiral galaxies, for example, is so forceful that the stars in the incoming discs scatter randomly. What is left is a blob-like galaxy known as an elliptical. So, spiral galaxies evolve into elliptical galaxies, and irregular galaxies are those in the process of becoming one or the other. Some galaxies grow; some are swallowed up by their larger neighbours, but most regenerate their stellar populations and so never really ‘die’.